In the parent application, incorporated by reference herein, a metal container is described which is of a ringless type. The container described in the parent application had several unique features, one of which included the use of bale ears or lugs into which the ends of a wire handle or bale were snapped for carrying the container. The ears are resistance welded to the container. This application relates to the manufacturing technique used in welding the lugs to any cylindrical container, whether of the ringless or ring type design.
Heretofore, the conventional method of manufacturing cylindrical metal paint containers, particularly of the gallon size, was to stamp the ear into a cup shape form, the cup having a base with an opening therethrough and a rim from which extended a circular flange. The metal, tin plated stock was stamped into a plurality of rectilinear sheets and each sheet was subjected to movement through several die forming stages which progressively indented the sheet to provide a recess for receiving the ears with the sheet stock then bent over the flanges of the ears to hold the ears in place. After the sheet was thus formed and clinched about the ears flange, the sheet was then formed into a cylindrical form and the width edge of the sheet crimped back onto itself for a soldered side seam or overlapped for a resistance welded side seam. The bottom of the container was then double seamed to one end of the cylinder and a ring double seamed to the other end of the cylinder in accordance with conventional practices.
Applying the ears to the container by clinching the container about the ears, while occurring at a fast rate is nevertheless a limiting factor in the capacity of a can manufacturing line resulting in a lesser production rate than that which is otherwise possible to achieve or alternatively requiring more than one ear applying station to maintain a high throughput capacity. Importantly, it has been determined that as the metal is stretched while it passes through the dies and is clinched about the flanges of the ears, the straightness of the width edges of the sheet is distorted. The straight edge distortion does not occur in a repeatable, consistent manner, but is irregular and when the longitudinal side seam is to be resistance welded, the overlap must be increased to compensate for the irregularity. In a ringless can design, the resistance welded side seam becomes pronounced and can adversely affect the seal of the lid. In a conventional can design, the overlap becomes of concern when the container is used to store water based paints. For water based paint applications the seam is coated to prevent rust by a spray striped system. When the overlap becomes significant, the coating cannot penetrate the space between the overlapped metal. An overlap of about 0.4 to 0.5 mm can be effectively penetrated with a spray striped system. Unfortunately, the distortion of the straight edge from clinching the ears typically can equal 1 mm. This fact generally dictates that resistance welding of the side seam is not possible for water based, metal paint containers which clinch the ears in the method described above and necessitates the more expensive, crimped soldered seam method.
The prior art has attempted to overcome such limitations and has applied the ears to the can bodies by projection welding. However, the ears have been applied after the body has been formed into a cylindrical shape and the longitudinal side seam welded. When the ears are applied to the cylindrical bodies, the fixture required to spot weld the ears to the rounded body limit the time required for the operation to about 20 or so can bodies per minute. This in turn limits the throughput of the line or alternatively requires a plurality of expensive ear welding stations. Attempts to projection weld the ears prior to forming the rectilinear sheet into a cylinder resulted in a flattening of the cylinder where the ears were applied or the ears "popping off" if they were not adequately welded.
It is also noted that the present state-of-the art within the metal can industry does not permit butt-welding of the container's longitudinal side seam but that efforts are being made to utilize laser technology to effect butt-welding. When such developments occur, it is quite likely that the straightness of the flat plate's width edges will have to be carefully controlled and the distortion of the edge's straightness by the ear clinching process will not be possible.